1. Field of the Invention
The present invention relates to a planar light source unit and a display device, and more particularly, to a planar light source unit and a display device having a light guide plate with an anisotropic diffraction grating, and a downward-pointing prism sheet.
2. Description of the Related Art
Liquid crystal display devices are widely used as image display devices for personal computers and various types of monitors. The liquid crystal display devices generally include a liquid crystal display panel and a backlight unit mounted on the back of the panel. The liquid crystal display panel displays images by controlling transmission of light. Among several types of liquid crystal display devices is one employing a sidelight (or edgelight) type backlight unit. A light source is positioned at the side of a display surface in this type of liquid crystal display device so as to make the device assembly as thin as possible.
The sidelight type backlight units generally include a light guide plate for guiding light from a light source and a reflection sheet placed on the back of the light guide plate. The backlight unit further has a diffusion sheet for diffusing light for uniform surface illumination and a prism sheet for collecting light from the light guide plate and directing the light vertically to the display surface.
There are two types of prism sheets: a downward-pointing prism sheet that is placed so that a prism surface where prism structures are formed faces toward a light guide plate, and an upward-pointing prism sheet placed so that the prism surface faces toward a display surface.
The prism sheet may be replaced by a hologram sheet to achieve more efficient use of light and higher front luminance. This technique is described in Japanese Unexamined Patent Application Publication H11-295713. A backlight unit according to this technique has a light guide plate, hologram sheet, and diffusion sheet. The hologram sheet is placed above the light guide plate with a certain space inbetween, and the diffusion sheet is placed above the hologram sheet. The hologram sheet bends the obliquely incident light from the light guide plate in the direction normal to a display so as to increase front luminance. The hologram sheet allows more efficient use of light than the prism sheet, thus achieving higher luminance.
Another known technique employs a backlight unit having a light guide plate with a scattering hologram on the top surface and a light diffusing pattern on the bottom surface. This technique is described in Japanese Unexamined Patent Application Publication No. 2002-231029. The scattering hologram is composed of a diffraction grating being integral with the light guide plate. The scattering hologram lets the light traveling through the light guide plate exit from the light guide plate. It bends the exiting light in the direction substantially perpendicular to the exit surface of the light guide plate and scatters the light.
The light having passed through the scattering hologram thus becomes the scattered light with the center light rays substantially perpendicular to the exit surface, and high light use efficiency is achieved while the light is scattered. Since the scattering hologram has both light collecting effect and light scattering effect, it eliminates the need for a prism sheet for collecting light on the front surface and a scattering sheet for scattering light, thereby increasing luminance. Japanese Unexamined Patent Application Publication No. 2002-231029 further describes a technique that employs an on-axis scattering hologram sheet on the top surface of a light guide plate. The on-axis scattering hologram sheet is able to scatter light, but unable to deflect light. Thus, a downward-pointing prism sheet is placed between the scattering hologram sheet and the light guide plate so as to bend light in the direction normal to a display.
Another known technique in which a light guide plate and a hologram are integrally formed employs a backlight unit having a light guide plate with a hologram pattern on the top surface and smooth prism structures on the bottom surface. The hologram pattern scatters light in the directions which are in-plane of the top surface of the light guide plate and perpendicular to the incident direction of light on the light guide plate. The prism structures on the bottom surface change the light path to let the light exit from the light guide plate at a given angle.
As described above, there are several known techniques for making a planar light source unit having a light guide plate with a hologram pattern, and a prism sheet. Not enough study, however, has been done on the relationships between particular prism sheets and light guide plates with a particular hologram pattern. Thus, not high enough light use efficiency and luminance have been achieved.